A decision made 70 years ago to reforest vast swathes of Japan with just two kinds of tree has come back to haunt the country. In February, videos showing what looked like waves of smoke blowing off an evergreen forest went viral in Japan. It wasn't smoke — it was pollen, and the videos were a warning to tens of millions of residents of the archipelago nation: prepare your masks and allergy medicine. 

Every spring (which is already arriving earlier in Japan due to climate change), you'll see people of all ages wearing masks on the streets of cities across the country. The reason: hay fever, driven by all the pollen. Hay fever — also known as allergic rhinitis — has now become a national crisis in Japan, with an estimated 43% of the population experiencing medium to severe symptoms. This compares to 26% in the UK and 12-18% in the US.